In order to get tree sap in Ark Survival Evolved you need to find the poison trees in the Bog Biome. Once you find the large glowing tree walk up to it to grab resources. Be sure there are no hostile creatures around first because the tree will blast poison at you. Once poisoned it will be very hard to see, just like if you get spit on by a dilo. Not only will you collect sap from the poison tree you will also find Congealed Gas Balls, Absorbent Substrate, and Bio Toxin. Tree sap is sometimes awarded after a mission is completed.
Poison Trees in Ark Genesis give Tree Sap
Some of the locations of the poison trees in Ark Genesis are: